That shellcode book might be nice but I think it's totally useless if you want to exploit your linksys. I'm not sure but I highly doubt the linksys is running some sort of I386 architecture. If you want to exploit a service on the linksys I suggest finding out what type of processor it has and what it's architecture is like. You would then need to get the firmware and disassemble it using the correct disassembler.

Another option is to see if the web interface has any flaws in it that would allow you to make changes without authentication.